ARM: dts: imx6sx: add pu power domain support

Add PU power domain support, GPU is the only
module inside PU power domain, and PU power
is supplied by LDO_SOC.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>